a. Field of the Invention
Broadly speaking, this invention relates to the measurement of distortion. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for measuring the intermodulation distortion generated by microwave repeaters, and the like.
B. Discussion of the Prior Art
Multi-channel, microwave transmission systems employing suppressed-carrier, single-sideband, amplitude-modulation have been proposed as a replacement for existing frequency-modulation systems. Due to the nature of amplitude-modulation, such systems will be more sensitive to non-linearities in the microwave repeaters than are the FM systems, for example. Of particular concern are third-order IM distortion products which tend to fall within the passband of the transmission system and, hence, are potentially troublesome.
The classic SMPTE method for measuring the IM distortion of audio amplifiers calls for the application to the amplifier's input of two fixed frequency test-tones, usually 400 Hz and 1000 Hz, and then examination of the amplifier's output for intermodulation products of frequencies such as 600 Hz, 1400 Hz, etc.
This classic approach may also be used with radio frequency and microwave amplifiers but the results that it yields are not too informative, due to the broad bandwidth of the devices under test. Accordingly, it is known to measure the non-linearity of such devices by applying to the amplifier two test-tones, f.sub.1, f.sub.2, of selectively adjustable frequency. In U.S. Pat. Nos. 3,411,080 and 3,369,176, for example, which respectively issued on Nov. 12 and Feb. 3, 1968, to A. O. Palatinus, the two test-tones are offset by a fixed frequency difference .DELTA.f, and the degree of distortion present in the amplifier's output signal determined by examining the magnitude of harmonics and subharmonics, (M.DELTA.f)/2; M = 1,2,3 . . . , of the offset frequency .delta.f. See also U.S. Pat. No. 3,241,059, to W. I. Wu.
The circuitry disclosed by Patatinus is complicated, expensive, and difficult to align.